The invention relates to a filter consisting of cavity resonators, in which filter the couplings between the resonators can be adjusted. A typical application of the invention is an antenna filter in a base station of a cellular network.
In order that the frequency response of a bandpass filter complies with the requirements, its passband must on the one hand be located at the right place on the frequency axis and on the other hand be of the right width. In a resonator filter this requires that the resonance frequency, or natural frequency, of each resonator is right and in addition the strength of the couplings between the resonators is right. In serial production, a filter consisting of cavity resonators is naturally formed by mechanical dimensions so that these requirements are realized as fully as possible. In practice, the manufacturing process is not precise enough, for which reason the filter must be tuned before adoption.
In tuning, both the natural frequency of the resonators and the strength of the couplings between the resonators are adjusted. The latter adjustment affects the bandwidth of the filter. Both adjustments can be implemented in many ways. The conventional way is to provide the structure with metallic tuning screws so that these extend into the resonator cavities and/or into the coupling openings between the resonators. When turning e.g. a tuning screw for coupling adjustment deeper to a coupling opening, which is located in the upper part of the filter, the strength of the coupling between the resonators in question weakens, which has the effect of narrowing the band. A flaw of the use of the tuning screws is that the junction between them and the surrounding metal can cause harmful passive intermodulation when the filter is in use. In addition, the electric contact in the threads can degrade in the course of time, which results in change in the tuning and increase in the losses of the resonators.
The strength of the coupling between two resonators can be adjusted also by means of a bendable tuning element arranged close to the coupling opening. The flaw of such a solution is that in a multiresonator filter the tuning elements may possibly have to be bent in several steps in order to achieve the desired frequency response. The lid of the filter has to be opened and closed for each adjustment, for which reason the tuning is time-consuming and relatively expensive.
FIGS. 1a and b present a way to adjust the strength of the couplings between the resonators of a filter, known from the publication U.S. Pat. No. 5,805,033. The filter comprises a conductive housing formed by a bottom 101, outer walls 104, and a lid 105, the space of which housing is divided into resonator cavities by conductive partition walls 112a-b. Two resonators 110, 120 of the filter are seen in FIG. 1a from above with the lid removed, and FIG. 1b shows the cross section of the filter at the partition wall of the resonators in question.
In the middle of each resonator cavity there is a cylindrical dielectric object for decreasing the size of the resonator, such as the dielectric object 111 of the first resonator 110 and the dielectric object 121 of the second resonator 120. The bases of the cylinder are parallel with the bottom 101 and lid 105 of the filter. The dielectric objects have been dimensioned so that a TE01 waveform (Transverse Electric wave) is excited in them at the use frequencies of the filter. Thus, the resonators are half-wave cavity resonators by type.
To implement the coupling between the resonators 110 and 120 there is an opening in their partition wall 112a-b, which opening extends from the lid to the bottom and narrows towards the bottom. To adjust the strength of the coupling there is a tuning element 115 in the coupling opening, which is a round metallic plate parallel with the lid 105. The plate has been fastened to the lid through a threading rod which extends outside the filter housing. When the threading rod is turned, the tuning element 115 moves vertically and changes the strength of the coupling between the resonators. In the figure the adjusting range of the tuning element is between the lower surface of the lid 105 and the plane represented by the upper part of the dielectric objects 111, 121. In this case, when the tuning element is insulated from the threading rod, the coupling becomes stronger when it is moved downwards, and vice versa. When the coupling strengthens, the resonance peaks of the resonator pair move away from each other, in which case the bandwidth increases.
A drawback of the solution described before is that the tuning of the bandwidth has been designed to be manual. The automatic tuning by using actuators is difficult to implement.